When energy is released into the subsurface from a seismic source or an acoustic source, a large portion of the energy encounters various geological strata or formations and is subjected to attenuation therein prior to being detected by a set of receivers. Detection by one or more receivers makes it possible, after performing multiple measurements and processing, to obtain an image of the subsurface structure. The signal y(t) detected by a receiver may be taken to be the response of the subsurface that the pulse p(t) has passed through after being emitted by an emitting source. More precisely, this response y(t) corresponds to the response c(t) of the subsurface plus added noise n(t), as shown diagrammatically in FIG. 1a. To illustrate this phenomenon, FIGS. 1b and 1c which are figures that have been obtained experimentally show the following: FIG. 1b, the emission of a single pulse p(t); and FIG. 1c, the detected signal y(t).
The greater the quantity of energy supplied by the pulse as emitted, the better the signal-to-noise ratio. Thus, it is generally desirable to provide as much energy as possible for each measurement to be made. This maximum energy is generally provided either by emitting pulses of short duration and high intensity, or else by emitting a series of pulses of lower intensity and over a relatively long period of time. The emission of high intensity pulses is not suitable in all applications. In particular, it is not suitable for down hole seismic surveys, and in particular for between-well seismic surveys. However, in both of these cases, the seismic sources, comprising explosive charges, for example, are received inside a well. It is not possible to use such sources capable of delivering sufficient energy without running the risk of damaging the wells. The same problem arises in acoustic prospecting.
It is then preferable to emit series of pulses from transducers which unfortunately have limited peak powers. Indeed, when these limits are exceeded, the transducers are damaged or even destroyed. In any event, the power transmitted via cables connecting the energy storage means (capacitors, for example) in the wells to external equipment is also limited.
On reception, processing is performed which comprises summing response signals for a given series of pulses to obtain a single response signal. This method is known as "stacking" and consists in taking the average of a plurality of signals. Unfortunately, using this averaging method considerably increases acquisition time, and this increase in time may be unacceptable for downhole seismic surveys and in particular for surveys between wells. Emitting a series of pulses makes it possible to increase the received signal level when sources of limited peak power are used since the energy is then taken from N emitted pulses that are separated by regular time intervals. The time interval Tm cannot be less than the total duration of the response of the subsurface c(t) to avoid mixing up successive responses in time. The total acquisition time thus becomes equal to NTm.
An attempt has been made to solve this problem when performing logging measurements by using pulses that are encoded. Reference may be made, for example to the state of the art as constituted by U.S. Pat. No. 4,326,129. That patent teaches using encoded pulses for logging measurements in order to reduce acquisition times that are too long when using the signal-averaging technique. That document describes using an energy source associated with control means to obtain pseudo-random sequences of energy pulses. These sequences are injected into the subsurface from the well and receiver(s) located in the well detect them after they have been reflected on the various underground formations encountered. The processing performed on the detected signal then consists in correlating the detected signal with the emitted signal. Consequently, the processing consists in performing correlation by means of a correlator. The subsurface response signal obtained by the correlation operation (between the emitted signal and the detected signal) has secondary lobes. Consequently, this processing gives rise to a poor signal-to-noise ratio. When the technique described in that patent is applied to between-well surveying, the signal-to-noise ratio becomes even more critical.
The state of the art constituted by U.S. Pat. No. 4,969,129 discloses improving the signal-to-noise ratio by exciting a source of m-sequence type encoded pulses. This solution has the advantage of not generating secondary lobes. M-sequences are periodic sequences in which the period is chosen in such a manner as to be greater than the subsurface response time. A first drawback of this solution comes from the fact that it is necessary to record the signals during at least one period, and consequently a great deal of memory space is required, since period duration is of the order of one second. A second drawback of the method comes from the fact that it is necessary for the emitter and for the receiver to be synchronized during at least two periods. This means that very accurate clocks must be used. For a one-second period, the accuracy covers two seconds.